If Earth became uninhabitable in the next fifty years, it would be possible, in theory, to relocate perhaps five to ten thousand people to Mars over that period. The red planet is the best site in the solar system because it is relatively nearby (as close as 40,000,000 miles away) and because it is the most earth-like planet. Mars has ice caps for water, carbon dioxide for plants, a reasonable amount of sunlight, and useful raw materials in the rocks. Mars has frozen water in the soil, a thin non-poisonous atmosphere, seasons, and 24-hour days.
Martian colonists could build extensive greenhouses to grow plants, which could provide food and oxygen for life support. The key to human survival on Mars, however, would be a reliable energy source. Since there is no coal, oil, or natural gas on Mars, sunlight would have to be that source. Because it is farther from the sun, Mars receives only half as much sunlight as Earth. Since it does have fewer clouds, Mars actually absorbs about the same amount of sunlight at its surface as Earth. This solar energy could be used to heat greenhouses, fabricate glass and steel, and make fuel from the Martian soil. There is even a remote chance that some geothermal heat source might be found near one of the huge Martian volcanoes.
If this all sounds too easy, don't be fooled: moving a few thousand people to Mars would be the greatest human endeavor of all time. Astronomers know that Mars is very cold with an average temperature of -90oF and that deadly ultraviolet radiation reaches the surface. Also, every pound you send to Mars will cost over $1 million in travel expenses. At that rate it would $150,000,000 to send me!
The plan would be to send robots first to find the best site for the colony and then to mine the resources and build the shelters. The few dozen astronauts would arrive a year later and they would set up the life support systems and test their safety shelters. After about a decade of testing, the first colonists, probably engineers and technicians, would be sent to expand the Mars base to receive several hundred more people.
Once the colony was fully operational, plans would go into effect for recycling all materials and for minimizing pollution. New energy and agricultural technologies would be necessary to insure the survival of the colony over the long term. If successful, the colony could then expand to handle thousands of new colonists.
Only a small percentage of the Earth's population could be sent to Mars over the first hundred years. So for the time being, we all need to do our part to protect Earth and its ecosystems. No matter what scientists could do to develop a Mars base, no place in our solar system will ever duplicate the rivers, forests, oceans, and beautiful clouds of Earth. It is your planet: treat it kindly.